1. Field of the Invention
The present invention relates to hybrid refractive/diffractive lens systems and particularly to a refractive/diffractive achromat which is especially suitable for use as a taking (or an objective) lens in inexpensive cameras such as single-use cameras and a method of producing diffractive surfaces for such lens systems.
2. Description Relative to Prior Art
In order to obtain photographs with good quality images, the lens that focuses the light must be well corrected for aberrations. It is not enough for the objective lens to be corrected for monochromatic aberrations. The lens must also be corrected for chromatic aberrations for a relatively broad range of wavelengths. For each color or wavelength of light incident on a refractive lens, the lens will have a different focal length. It is this property of the lens that give rise to (longitudinal and lateral) chromatic aberrations. Currently, the objective lenses for cameras correct chromatic aberrations by using additional lens elements. However, this creates additional bulk and makes the lens system heavier and more expensive. These considerations are especially important for single use cameras which need to be light weight, compact and inexpensive.
Single-use cameras typically include a one or two element lens utilized at a large F/# so they can be used In a fixed focus mode where everything from two meters to infinity is nearly in focus. Single-use cameras of a single lens element type typically are not corrected for chromatic aberrations, which all singlets tend to suffer from. Lenses used for single-use cameras generally have relatively high levels of monochromatic and chromatic aberrations. Some of the monochromatic aberrations can be corrected in a plastic molded singlet element through the use of aspheric surfaces. However, at some point, the chromatic aberrations will be significantly worse than the monochromatic aberrations therefore limiting the minimum spot size. The resulting unachromatized images can also exhibit color fringing.
Current single-component objective lenses used in single-use cameras are made of low dispersion, low index of refraction materials (usually plastic) to minimize longitudinal chromatic aberration. Thus, in order to reduce the difficulty of correcting for chromatic aberration in a single-element lens system, lens designs have been driven in the direction of reducing dispersion (using low index, high Abbe number glass) in order to obtain the necessary power and reduce the numerical aperture (NA) of the lens. Higher curvature, thicker lenses have therefore been required. Such thicker lenses give rise to manufacturing errors since they are more sensitive to variations in lens thickness, wedge, tilt, and decentering.
Additional lens elements are used to provide chromatic aberration correction in multi-element, more costly, lens systems. When a cemented doublet (comprised of a positive and a negative power lens element) is used to correct for chromatic aberrations, a negative power lens element made of flint glass (i.e. glass having a low Abbe number) is cemented to the positive lens element which is typically made from a crown glass. However, because the negative lens element increases the focal length, the positive lens element is made stronger to compensate for that change in order to keep the original focal length. In order to obtain the necessary power, the positive lens element will thus need to have stronger radii of curvature and to be thicker. Such lenses also sacrifice weight and size in order to accommodate surfaces and elements which compensate for chromatic aberration. Alternatively, two air spaced, roughly symmetrical, lens elements separated by an aperture stop can also be used to get a better system performance. However, an additional element again increases weight and size of the system. Finally, when designing a single element optical system, a designer may use low dispersion glasses that still have a high index of refraction. However, such glasses are expensive.
Although various patents and publications have discussed the use of diffractive elements to compensate for chromatic aberration (see U.S. Pat. No. 4,768,183, U.S. Pat. No. 5,117,433, U.S. Pat. No. 5,044,706, U.S. Pat. No. 5,078,513, U.S. Pat. No. 5,117,306, U.S. Pat. No. 5,161,057, U.S. Pat. No. 5,208,701, and U.S. Pat. No. 5,229,880), designs for objective or taking lenses in single element cameras have not had any chromatic correction and typically have relatively steep surface curvatures. As previously mentioned, in order to avoid these and other problems, some single-use camera lens systems include two lens elements separated by an aperture stop. Similarly, consumer camera lenses in visible light applications, such as for taking photographs of friends, relatives or nature, use multiple lens elements to correct for chromatic aberrations.
Finally, several methods for manufacturing diffractive surfaces are known. A diffraction profile may be manufactured by a binary method, i.e. a "step function" method by etching the surface while applying consecutively two to four masks.
A "step function" method also results in alignment errors which result in inaccurracies introduced in a diffractive surface profile. The errors are introduced because the manufacturing process requires that each mask level be aligned with respect to the other. For example, a single zone comprised of 16 steps is made with four masks. Each of the four masks has to be aligned with respect to the others. Although such masks are aligned to each other to within a fraction of micron, the alignment errors nevertheless cause decreased diffraction and introduce wavefront errors. At the present time, there is no known method resulting in a perfect alignment of the masks.
Alternatively, a diffractive surface or a mold for manufacture of such surface may be cut using a diamond turning method.
Typically, diamonds used in diamond turning of Fresnel-like surfaces are fabricated with radii of 150 to 100 um or with a diamond having a flat of 3 .mu.m or more. This radius helps to reduce surface roughness and increase lifetime. However, the radius on the tool limits the sharpness of the corners. The decreased sharpness in the corners leads to more scattering and more undiffracted light. Also, in many instances the zone spacings one is required to fabricate are smaller than the radius of the diamond tips. Therefore, it would be impossible to fabricate them with this type of a rounded diamond tip. A diamond tip with a flat of 3 .mu.m or more also reduces the efficiency of the optical system and increases scattering.